// Package mailer is a thin SMTP wrapper used by the auth flows.
//
// The stdlib `net/smtp` is sufficient for our needs (password reset +
// email verification — low volume, plain-MIME). We keep one interface
// (Mailer) so handlers can be tested with an in-memory implementation
// and so we can swap to a transactional provider later (Resend, SES)
// without touching the call sites.
package mailer

import (
	"fmt"
	"net/smtp"
	"strings"

	"go.uber.org/zap"
)

// Mailer is the small surface auth services depend on.
type Mailer interface {
	Send(to, subject, htmlBody string) error
}

// Config carries the per-environment SMTP knobs.
type Config struct {
	Host     string
	Port     int
	Username string
	Password string
	From     string // "Name <addr@host>" or just "addr@host"
}

// Configured returns true if enough SMTP creds are present to send mail.
// Used by the auth service to decide whether to send-or-log.
func (c Config) Configured() bool { return c.Host != "" && c.From != "" }

// SMTP is the production Mailer. Connects per call (low volume → fine).
type SMTP struct {
	cfg    Config
	logger *zap.Logger
}

// New returns an SMTP mailer if creds are present, otherwise a NoopLogger
// that logs the message body to zap (useful in dev — the password-reset
// link shows up in your server logs).
func New(cfg Config, logger *zap.Logger) Mailer {
	if !cfg.Configured() {
		logger.Warn("SMTP not configured — mailer will log messages instead of sending")
		return &NoopLogger{logger: logger}
	}
	return &SMTP{cfg: cfg, logger: logger}
}

// Send delivers a single HTML email. Plain-text alternative is omitted —
// the templates are short and modern clients render HTML fine.
func (m *SMTP) Send(to, subject, htmlBody string) error {
	addr := fmt.Sprintf("%s:%d", m.cfg.Host, m.cfg.Port)
	auth := smtp.PlainAuth("", m.cfg.Username, m.cfg.Password, m.cfg.Host)

	var msg strings.Builder
	msg.WriteString("From: ")
	msg.WriteString(m.cfg.From)
	msg.WriteString("\r\n")
	msg.WriteString("To: ")
	msg.WriteString(to)
	msg.WriteString("\r\n")
	msg.WriteString("Subject: ")
	msg.WriteString(subject)
	msg.WriteString("\r\n")
	msg.WriteString("MIME-Version: 1.0\r\n")
	msg.WriteString("Content-Type: text/html; charset=UTF-8\r\n\r\n")
	msg.WriteString(htmlBody)

	// Most relays on port 587 want STARTTLS + AUTH. Port 465 = implicit TLS,
	// not handled here — add only if needed.
	if err := smtp.SendMail(addr, auth, fromAddr(m.cfg.From), []string{to}, []byte(msg.String())); err != nil {
		return fmt.Errorf("smtp send: %w", err)
	}
	return nil
}

// fromAddr extracts the bare address from a "Name <addr>" string. SendMail's
// `from` argument is the envelope-from, which must be the bare address.
func fromAddr(s string) string {
	if i := strings.LastIndex(s, "<"); i >= 0 {
		if j := strings.LastIndex(s, ">"); j > i {
			return s[i+1 : j]
		}
	}
	return s
}

// NoopLogger satisfies Mailer by writing the message into the application
// logger. Use it in development so you can copy reset links from the console.
type NoopLogger struct{ logger *zap.Logger }

func (n *NoopLogger) Send(to, subject, htmlBody string) error {
	n.logger.Info("📧 mailer (not sent — SMTP unconfigured)",
		zap.String("to", to),
		zap.String("subject", subject),
		zap.String("body", htmlBody))
	return nil
}